1. Field of the Invention
The present invention relates to compositions based on vinylidene chloride copolymers stabilised against heat and to their use. It relates more particularly to compositions based on vinylidene chloride copolymers stabilised against heat by the use of an epoxide stabiliser and to their use for extrusion of films and sheets intended for packaging.
2. Description of the Related Art
Vinylidene chloride copolymers, usually denoted by the acronym PVDC, form well-known impervious thermoplastic copolymers containing approximately 70 to 96% by weight of monomer units derived from vinylidene chloride, the essential comonomers usually consisting of vinyl chloride and (meth)acrylic esters and/or nitriles. Their high imperviousness, especially to oxygen, makes them materials of choice for the manufacture of films and sheets intended for packaging, in particular in the alimentary field.
It is also well known that PVDCs suffer from inadequate heat stability and that their heat stability is proportionately lower the higher their vinylidene chloride content (and hence their imperviousness). It is therefore indispensable that effective heat stabilisers are added to them before any melt-processing operation such as extrusion.
Many heat stabilisers for PVDCs, generally of the "acid-acceptor" type have already been proposed. These include inorganic compounds such as magnesium oxide and hydrotalcites. These compounds have the disadvantage of resulting in an opacification of articles made of PVDC, so that these inorganic stabilisers have to be ruled out in any application requiring a high transparency.
Epoxy compounds form another known class of effective heat stabilisers for vinylidene chloride copolymers, which do not exhibit the abovementioned disadvantage. The most commonly recommended of these are epoxidised oils, such as epoxidised soya oil and epoxy resins, more particularly liquid epoxy resins consisting essentially of bisphenol A diglycidyl ether. Epoxidised oils, such as epoxidised soya oil, are suitable for stabilising compositions intended for the manufacture of transparent articles However, they have the disadvantage of considerably reducing the imperviousness of the PVDCs in which they are incorporated. Furthermore, the use of liquid epoxy resins of the abovementioned type presents a problem in some countries because of regulations and/or directives imposing very severe limits on the migration and on the residual contents of bisphenol A diglycidyl ether. Recourse to solid epoxy resins derived from bisphenol A, which are less rich in epoxy groups, could possibly attenuate these migration problems, but at the expense of the efficiency where heat stability is concerned or of the imperviousness in the case of increase in the quantities of solid epoxy resin.
It has also been proposed to improve the heat stability of PVDCs by incorporating glycidyl methacrylate into the PVDC structure ("internal" stabilisation). In German Patent Application DE-A-25 23 769 in the name of Kanto Denka Kogyo there is a description of the manufacture of PVDC with improved heat stability by combined use of 0.5 to 10% by weight of glycidyl methacrylate in the aqueous suspension copolymerisation of vinylidene chloride and of other polymerisable vinyl monomers. In the best of cases, assuming the use of 10% by weight of glycidyl methacrylate at the beginning of the copolymerisation and the complete absence of hydrolysis of the epoxy groups during the copolymerisation, such copolymers would contain approximately 1% by weight of oxirane oxygen, which corresponds to an epoxy value of approximately 0.06. In this case, too, the improvement in heat stability takes place at the expense of the imperviousness of the PVDCs.
The objective of the present invention is to provide compositions based on vinylidene chloride copolymers stabilised against heat by the use of novel epoxide heat stabilisers which are particularly effective and which do not exhibit any of the abovementioned disadvantages.